This invention relates to an extruder screw and to an extruder using such a screw.
It is well known, in the rubber and plastics industries, to use extruders incorporating a screw rotatable within an elongate cylindrical housing. The screw conventionally has an intake section, a transition or mixing section where material is caused to pass through a small clearance between screw and housing to subject it to a high rate of shear in a short period of time, and a discharge section. For use with rubber elastomers the mixing section causes a decrease in the viscosity of the material passing through it with the object of producing an extrudate homogeneous in respect of constituents and temperature. For use with plastics materials the mixing section is used to complete the melting of solids from the intake section with the object of producing a high quality extrudate devoid of any solids. Known extruders achieve these objects with a varying degree of success.
In one known form of screw extruder the small clearance is provided by an annular ring dam located transverse to the axis of the screw, rotating with the screw, and having a small peripheral clearance with the housing. This has the advantage of being simple but has the disadvantage of forming a severe constriction with localised and relatively intense heat generation at the dam.
In an alternative known form, some screw flights are provided which operate with a clearance-fit in the housing and are specifically designed for the flow of material over them, and other screw flights are provided which operate with a running-fit in the housing and are not specifically designed to allow flow over them. In use, the running-fit flights cause material to be forced through the clearance between clearance-fit flights and housing. With this form, the constriction in the extruder is not so severe, operating pressures are lower and heat generation more diffuse. The present invention is concerned with improvements, chiefly improvements in simplicity and ease of manufacture, in this alternative known form. It also provides a screw giving a high standard of homogeniety prior to the discharge section so that, when used for plastics, the risk of unmelted solids passing to the discharge section is greatly reduced and, when used for rubber, a high level of thermal homogeniety can be achieved.